1. Field of the Invention
The invention relates to antistatic water-based, latex pressure sensitive adhesives using inherently low tribocharging adhesive polymers to provide protection for sensitive electronic devices. Low tribocharging adhesive tapes for use in microfabrication processes are provided.
2. Description of the Art
The use of acrylic polymers as pressure sensitive adhesives is well known in the art. Early preparation of acrylic adhesive compositions relied on solution polymerization techniques using significant quantities of organic solvents. Current trends demand severe reduction in solvent usage for the benefit of the environment and also to curtail costs incurred with expensive solvents.
Alternative methods to solution polymerization include suspension and emulsion polymerization. Both may be used to provide copolymer pressure sensitive adhesives. In emulsion polymerization a reaction occurs in micelles or emulsion microdrops suspended in aqueous medium. Any heat generated in the microdrops or micelles is quickly moderated by the effect of the heat capacity of the surrounding water phase. Emulsion polymerization proceeds with better control of exothermic reactions, and the resulting adhesive composition is non-flammable as the aqueous medium is the dominant component.
Offsetting these advantages is the energy required to evaporate water from the coated latex adhesives. The heat required is about five times the quantity typically used for a comparable solution adhesive. Less water in the adhesive would reduce the heat requirement. A reduction in water is likely, however, to lead to undesirable thickening of the emulsion. Ideally, a high solids, low viscosity emulsion will provide a variety of benefits. Unfortunately, sensitivity to moisture is a serious problem. Latex adhesives contain surfactant materials that attract water. Once affected by water, the adhesive may become cloudy, losing both adhesion and shear strength. A means to eliminate moisture sensitivity would yield superior adhesives for applications involving exposure to quantities of water, especially pressurized jets of water typically used in microfabrication processes.
European published application EP 0554832A discloses a water-based, high solids, moisture insensitive, latex pressure sensitive adhesive which has low coating viscosity as well as high shear, high compliance and controlled adhesion build-up. Suitable for a wide range of applications, this adhesive composition is however, not suitable for use with sensitive electronic devices that are subject to damage by electrostatic discharge. Use of antistatic adhesives in microfabrication processes reduces the incidence of electrostatic charge build-up followed by discharge in the proximity of sensitive electronic devices. Such discharge is known to damage devices rendering them useless for their intended function which adds to the failure rate and associated costs.
Microcircuit fabrication processes use pressure sensitive adhesive products at various stages of manufacture. With advances in miniaturization, circuit components became more susceptible to damage by electrostatic discharge. Therefore any source of electrostatic discharge, in the microcircuit fabrication process, could result in faulty microdevices. In wafer dicing operations, the required combination of tape characteristics includes good adhesion, water resistance, clean removability and lack of static discharge, a combination not easy to achieve.
Adhesive polymers have traditionally been a source of static charge. U.S. Pat. No. 5,378,405, incorporated herein by reference, was the first to disclose non-tribocharging, pressure sensitive adhesives wherein the adhesive polymer itself, in microparticulate form, is conductive.
U.S. Pat. No. 5,508,107 extends the application of antistatic microparticulate adhesives to operations that convert semiconductor wafers, supporting multiple integrated circuit chips, into individual chips. No evidence exists in the prior art for inherently low-tribocharging latex adhesive compositions.
A number of methods are known for preparing antistatic adhesive compositions by adding conductive moieties to conventional adhesive formulations. Antistatic species may be introduced as conductive materials such as electrically conductive metal or carbon particles. Compositions of this type are disclosed in various references including EP 0276691A, EP 0518722A, U.S. Pat. No. 4,606,962, EP 0422919A, U.S. Pat. Nos. 3,104,985, 4,749,612 and 4,548,862.
The addition of ionic materials to reduce generation of static charge is also known. Suitable materials of this type include ion conducting species such as those disclosed in Japanese patents JP 61,272,279 and JP 63,012,681.
Adhesive tapes of the present invention extend the scope of the antistatic adhesive concept to a broader range of adhesives. They provide latex adhesives which, coated on suitable substrates, prevent build up of static charge during use in microfabrication operations and similar situations requiring freedom from electrostatic discharge. Although disclosures in U.S. Pat. No. 5,378,405 suggest that latex formation is undesirable to the preparation of antistatic adhesives, the current inventors discovered adhesive latex compositions that fully satisfy requirements. The effect is so pronounced that tapes of the current invention are essentially non-tribocharging, even at low humidities.
Further, adhesive formulations of the invention and tapes made therewith exhibit transparency. In certain embodiments, optical clarity is exhibited.
Further, adhesives formulations of the invention can be designed with a variety of adhesion levels, depending on what is desirable for the specific application. For microfabrication applications, low adhesion is usually desirable, e.g., less than about 22N/100 mm. For some applications, adhesion values as low as 3.5N/100 mm are desired.
Latex adhesives differ from microparticulate adhesives, as described in U.S. Pat. No. 5,378,405, in the characteristics of the adhesive layers produced by coating the adhesives on suitable substrates. Typical latex compositions form uniform layers when polymerized microdrops coalesce, as water evaporates from the coating, during drying. Microparticles, on the other hand, do not possess sufficient fluid character for interparticulate coalescence. As a result, microparticle coatings contain individual particles which tend to cause relative surface roughness and opacity when compared to the transparent, planar-surfaced latex adhesive coatings.